Enzymes are biological catalysts (biocatalyst) which bring about chemical reactions in living cells. They accelerate the rate of the reaction and are usually present in very small amount in various cells. They can also exhibit their activity when they have been extracted from the source. They are organic compounds and a number of them have been obtained in crystalline form. Today, more than 840 enzymes are known.

Vitamins are organic nutrients that are required in small quantities for a variety of biochemical functions and which, generally, cannot be synthesed by the body and must therefore be supplied in the diet. The first discovered vitamins, A and B, were found to be fat and water soluble, respectively. As more vitamins were discovered, they were also shown to be either fat or water soluble, and this property was used as a basis for their classification.